[T]he production of heat alone is not sufficient to give birth to the impelling power: it is necessary that there should also be cold; without it, th… - Nicolas Carnot

There is almost as great a distance between the first apparatus in which the expansive force of steam was displayed and the existing machine, as between the first raft that man ever made and the modern vessel.

Wherever there exists a difference of temperature... it is possible to have also the production of impelling power. ...All substances in nature can be employed for this purpose, all are susceptible of changes of volume, of successive contractions and dilatations, through the alternation of heat and cold. All are capable of overcoming in their changes of volume certain resistances... A solid... A liquid... An aeriform fluid... If it is enclosed in an expansible space, such as a cylinder provided with a piston, it will produce movements of great extent. Vapors of all substances capable of passing into a gaseous condition, as of , of mercury, of , etc., may fulfil the same office as vapor of water. ...Most of these... have been proposed, many even have been tried, although... without remarkable success.

Notwithstanding the work of all kinds done by steam-engines... their theory is very little understood, and the attempts to improve them are still directed almost by chance.